As a laser source having broad application prospects, the fiber laser has advantages of a tunable bandwidth, a high signal-to-noise ratio, and a narrow output laser linewidth, and can be widely used in fields such as fiber sensing, optical fiber communication, and optical processing. The fiber laser comprises three parts of a pumping source, a resonator cavity and a gain medium. The longer the cavity of the fiber laser is, the greater the nonlinear effect of the fiber laser will be. Thus it is necessary to shorten the length of the fiber. Meanwhile, short cavity is an important prerequisite to achieve single longitudinal mode operation for fiber lasers. Short cavity fiber lasers have simple structures and are easy to be implemented. Short cavity fiber lasers typically consist of a pair of fiber gratings, and a gain medium connected therebetween, and this structure is called a Distribute Bragg Reflection (DBR) type fiber laser. Short cavity fiber lasers are usually used to generate narrow linewidth laser output. NP Photonics, a U.S. company, utilized a 2 cm long erbium-doped phosphate glass fiber DBR laser to obtain a laser output with the power of 100 mW and the linewidth of 2 kHz. In 1992, Ball and others achieved a 1548 nm single frequency output with a linewidth of 47 kHz using a 980 nm LD pump source by adding two Bragg gratings into the two ends of the 50 cm long Er3+ doped fiber for the first time. The two Bragg gratings are 1.25 cm long with the same Bragg wavelength, and have reflectance of 72% and 80% respectively. In 2007, A-FR company developed a type of fiber laser with the cavity length less than 5 cm, linewidth less than 3 kHz and output power up to 150 mW.
The short cavity fiber laser has several advantages such as a few numbers of longitudinal mode output, and stable output with no mode-hopping phenomena, and it is often used in the field of fiber sensing. Therefore, there will be important theoretical significance and application value to designing a sensing system based on longitudinal mode output by the short cavity fiber laser.
In accordance with the physics definition, the magnetic field is a special form of field existing in space around the current, moving charge, magnetic or variable electric field. The measurement of the magnetic field is to sense the existence of the substance and to determine parameter values of the substance by experimental means, which is not only essential to the measurement of the magnetic, but also widely used in other fields. As the nature and intensity of the measured magnetic field vary greatly, the measuring methods are various. With the development of science and technology, especially discovery of new effect and new phenomenon in the field of solid state physics, methods for measuring the magnetic field, and the sensitivity and accuracy of measurement made great progress. The development of electronic technology and computer technology has greatly changed measurement of a magnetic field in the fields of automation and digitalization. Commonly used methods for measuring the magnetic field include a) current method: determining the magnetic field by measuring current based on the strict relationship between the current producing the magnetic field and the magnetic field; b) electromagnetic induction method: measuring the magnetic field utilizing Faraday's law of electromagnetic induction; and c) measuring the magnetic field utilizing magnetic effects of some materials (e.g. Hall Effect). Commonly used magnetic field measurement instruments include the electromagnetic induction magnetic field measurement instrument, the Hall Effect magnetic field measurement instrument, the magnetoresistive effect magnetic field measurement instrument, magnetic resonance magnetic field measurement instrument, and magneto-optical effect magnetic field measurement instrument.
Therefore, there is a need for a method and system for accurately measuring magnetic field utilizing the features of the short cavity fiber laser.